The present invention relates to an electrophotographic toner and more particularly to an electrophotographic toner to be used for an image forming apparatus such as an electrostatic copying apparatus, a laser beam printer or the like.
In the image forming apparatus above-mentioned, the surface of a photoreceptor is exposed to light to form an electrostatic latent image on the surface of the photoreceptor. A developer containing an electrophotographic toner and a carrier is let come in contact with the surface of the photoreceptor. The electrophotographic toner is electrostatically stuck to the electrostatic latent image, so that the electrostatic latent image is formed into a toner image. From the photoreceptor surface, the toner image is transferred to and fixed on paper. Thus, an image corresponding to the electrostatic latent image is formed on the paper surface.
As the electrophotographic toner above-mentioned, there may be used one as obtained by blending a fixing resin with a coloring agent such as carbon black or the like, an electric charge controlling dye and the like and by pulverizing the blended body into particles having sizes in a predetermined range.
It is known that the electric charging characteristics of such an electrophotographic toner greatly depend on a surface dye density which refers to the amount, per one gram of toner particles, of the electric charge controlling dye which is exposed onto the surfaces of toner particles and which contributes to the generation of an electric charge.
To improve the electric charging characteristics, there has been proposed an electrophotographic toner improved in surface dye density to the range from 4.0.times.10.sup.-3 to 9.0.times.10.sup.-3 g/g as compared with the conventional range from 2.0.times.10.sup.-3 to 4.0.times.10.sup.-3 g/g (Japanese Patent Unexamined Application No. 36757/1986).
The surface dye density is obtainable in the following manner. That is, the dye present on the surfaces of toner particles is selectively extracted by a solvent such as methanol or the like which dissolves only the electric charge controlling dye, and the solution thus extracted is measured by an absorbance measuring method or the like to obtain the amount of the extracted dye, which is then converted into the amount of dye per toner of 1 gram.
It is found that, when a conventional electrophotographic toner including a toner improved in surface dye density is repeatedly used for a long period of time in a high-speed-type image forming apparatus in which the image forming speed is high, the developer is lowered in electric charging characteristics, causing troubles such as "forward flow", toner scattering, unstable image density and the like. The term of "forward flow" refers to a phenomenon that an excessive amount of toner electrostatically stuck to an electrostatic latent image due to low electric charging characteristics, is rubbed by a magnetic brush of a developing device and flows forward in the image forming direction.
Upon study of the reasons of the troubles above-mentioned, the following has been made clear. In a high-speed image forming apparatus, the developer is stirred under severer conditions than in a normal image forming apparatus. Accordingly, when the developer is repeatedly used for a long period of time, the dye exposed onto the surfaces of toner particles falls off therefrom to deteriorate the carrier. This lowers the entire developer in electric charging characteristics, thus causing the troubles above-mentioned.
Upon study from another point of view, the following has been made clear. In a conventional electrophotographic toner, the toner-surface presence rate of electric charge controlling dye, i.e., the rate of the amount of a dye present on the surfaces of toner particles to the total amount of the dye, is as high as 30 to 90% by weight. This means that a great amount of electric charge controlling dye is exposed to the surfaces of toner particles. Accordingly, in a high-speed image forming apparatus, the dye exposed to the surfaces of toner particles falls off therefrom as mentioned earlier, thus deteriorating the carrier. Thus, the entire developer is lowered in electric charging characteristics.
On the other hand, the electrophotographic toner is prepared by dispersing and mixing toner components such as a fixing resin, a coloring agent, an electric charge controlling dye, a releasing agent (off-set preventive agent) and the like, and by melting and kneading the resultant mixture, which is then pulverized and classified.
At the pulverizing and classifying steps, there is generated fine powder of which size does not reach a predetermined one. This greatly lowers the material yield. To improve the material yield, as shown in a flow chart in FIG. 3, such fine powder is reused as added to toner materials before the toner materials are dispersed and mixed.
More specifically, the respective components forming an electrophotographic toner, such as a fixing resin, a coloring agent, an electric charge controlling dye, a releasing agent (off-set preventive agent) and the like are blended in a predetermined blending proportion together with fine powder, and then dispersed and mixed with each other (step 1).
The resulting mixture is then molten and kneaded (step 2), and the resultant molten and kneaded body is cooled and solidified, and the resultant solidified body is subjected to coarse pulverizing, fine pulverizing and classification (steps 3 to 5), thus producing an electrophotographic toner having a predetermined particle size.
However, when the toner thus produced with fine powder reused as above-mentioned (hereinafter referred to as fine-powder regenerated toner) is used for a two-component developer, the following troubles are caused.
1) The amounts of consumed and collected toner are increased, thereby to lower the transfer efficiency.
2) Toner scattering contaminates the inside of an image forming apparatus, resulting in contamination of a reproduced copy due to toner falling.
3) A formed image blots.
4) In a formed image, gradation is lost so that the image tone becomes hard.
Upon study of the reasons of why the conventional fine-powder regenerated toner presents the problems above-mentioned, the following has made clear.
In a normal toner production method, at the step of dispersing and mixing the respective components, the component particles are finely pulverized and uniformly mixed upon reception of a shear force generated by mixing. However, when fine powder is added to the components before they are dispersed and mixed, the fine powder serves as a sliding material and therefore prevents the components from being pulverized by a shear force. Accordingly, the components cannot be sufficiently finely pulverized but remain in the form of relatively large lumps. In particular, the electric charge controlling dye incompatible with the fixing resin remains in the form of large lumps even in the subsequent melting and kneading step. Accordingly, on the surface of the fine-powder regenerated toner thus produced, the electric charge controlling dye is present in the form of relatively large lumps which are liable to readily fall off from the toner particles.
Accordingly, when the fine-powder regenerated toner as above-mentioned is repeatedly used together with a carrier in an image forming process for a long period of time, the electric charge controlling dye falls off from the toner particles to contaminate the carrier, thereby to deteriorate the electric charging characteristics of the developer in its entirety. Thus, the troubles above-mentioned are caused.
Alternately, it is proposed to lengthen the dispersing and mixing period of time as compared with a conventional period of time in order to promote the pulverization of the components. However, since the added fine powder serves as a sliding material, the expected effect cannot be produced. On the contrary, as the dispersing and mixing period of time is lengthened, the productivity is accordingly decreased.